Short focus lens at focal point of long focus lens



A. J. GALE ETAL Filed Jan. 2, 1959 mcus DEFOCl/S 4 +W Aug. 17, 1965 m fI/lI/I/IlI/II/III/IIIIIIA United States Patent 3,201,631 SHQRT FOCUS LENS AT FOCAL POINT OF LONG FOCUS LENS Alfred J. Gale and John C. Nygard, Lexington, Mass, assignors to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed .lan. 2, 1959, Ser. No. 784,573 2 Claims. (Cl. 313-84) This invention relates to the focusing of beams of charged particles and in particular to the application of alternating gradient focusing to varying potential high voltage accelerators. The invention is particularly useful in connection with X-ray generators wherein it is desired to obtain a fine focal spot at the target. The invention, therefore, comprehends X-ray generating equipment comprising a linear electron accelerator in combination with a magnetic focusing assembly for focusing the beam on the target which focusing assembly comprises two or more quadrupole magnetic lenses, successive lenses being of opposite polarity and the lenses having relative axial lengths so chosen as to insure that the beam is brought to approximately the same focal point in the two principal planes which focal points are coincident with the target. Such a magnetic focusing assem bly is hereinafter called an alternating-gradient magnetic focusing system and the equivalent electrostatic focusing assembly is hereinafter called an alternatinggrad'ient electrostatic focusing system. The invention also comprehends a method of obtaining a line focal spot comprising an alternating gradient magnetic or electrostatic focusing system of relatively long focal length and of relatively small angular aperture, in combination with. an alternating gradient focusing system of relatively short focal length and of larger angular aperture, in which the second focusing system is placed in the neighborhood of the focal point of the first focusing system; by the focal point of the first focusing system we mean that point at which the spot size in the plane of the second focusing system varies symmetrically about a minimum for the maximum useful energy range.

While in principle the invention is not limited to any particular type of charged particle beam, it is principally advantageous when used with electron beams which in turn are employed for the production of X-rays, nor is the invention limited to any particular type of electron accelerator for this purpose. However, for illustrative purposes the invention will be described with reference to X-ray generators wherein the electron beam is produced by a linear electron accelerator.

The beam of electrons from a linear electron accelerator can be very suitable for the production of X-rays for industrial radiography excepting for cross-sectional size of the beam. in general, the beam from a linear accelerator is of circular cross-section, and is usually of the order 0.5 to 1 cm. diameter. The edges of the beam are not sharply defined, and conventionally the diameter of the beam is given as that diameter hole through which half the beam current will pass. If such a beam impinges on a target, and the X-rays resulting are used for radiograp-hy, the relatively large size of source results in unsharpness in the radiographs, unless the object is placed at a considerable distance from the target, which necessitates longer exposures than would otherwise be required. If the spot size were to be reduced, the object could be brought nearer, with a consequent shortening of exposure time. However, there is no point producing a spot size of less than about 1 mm. diameter, because of inherent unsharpness due to scatter in the film, etc.

One way of obtaining a small spot size is to collimate the electron beam at the gun. However, collimatingthe beam to a small diameter by this method results in considerable reduction in output with consequent increase in xposure times.

One purpose of this invention is to provide a means of focusing the electron beam from a linear accelerator to a small diameter with relatively little loss of beam current.

According to the present invention X-ray generating equipment comprehends a linear electron accelerator in combination with a magnetic focusing assembly for focusing the beam on the target which focusing assembly comprises two or more quadripolar magnetic lenses, successive lenses being of opposite polarity and the lenses being of different axial lengths and/or magnetic field strengths to satisfy the requirements that the beam should be brought to approximately the same focal point in the two principal planes which focal points are coincident with the target.

According to a preferred arrangement there are two quadripolar magnetic lenses and the first lens is of shorter axial length and/0r magnetic field strength than the second.

The preferred shape of the pole pieces of the magnetic lens is that of a segment of a right hyperbolic cylinder, although in practice a sufliciently close approximation can be found in the shape of a segment of a right circular cylinder. For such a lens, the transverse magnetic field strength across the principal planes is directly proportionalto distance from the axis.

It has been found that with such an arrangement it is possible to reduce the cross section of the beam or the target without appreciable loss of power and without introducing noticeable astigmatism.

The quadripolar magnetic lenses preferably employ permanent magnets though electro magnets could be used.

Theoretically, for the preferred arrangement, there Will be no astigmatism if [cos X-Zsin X] cosh y-f sin X sinh Y A [cos X-Z Sin X] sinh Y-A1 sin X cosh Y [cosh X+Zsinh X] cos Y+ sinh X sin Y 2 [cosh X-l-Z sinh X] sin YA sinh X cos Y Where Similar expressions can be obtained for the conditions for astigmatism when more than two magnets are used,

. but the case of two magnets is the one of greatest interest.

such a lens has sufiiciently small astigmatism to enable a beam of electrons from a 4 m.e.v. linear accelerator, initially of 1 cm. diameter, to be focused to a spot of approximately 2 mm. diameter.

In order that the invention may be more clearly understood reference Will now be made to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of a linear accelerator incorporating a focusing arrangement;

FIG. 2 is a cross-sectional view showing a magnet arrangement in detail;

FIGS. 3a and 3b illustrate the effect of focusing;

FIG. 4 is a diagram illustrating apparatus similar to that shown in FIGS. 1, 2 and 3, but which includes two alternating gradient focusing systems in accordance with the invention;

FIG. 5 is a somewhat diagrammatic view showing one form of alternating gradient focusing system of a simplified nature and FIG. 6 is a sectional view along the line 6-6 of FIG. 5.

In FIG. 1 the reference 1 indicates the linear accelerator generally, 2 and 2' are quadripolar magnet assemblies, 3 is the target and 4 is a collimator.

FIG. 2 shows a magnet assembly in which 5 is a yoke and four magnets 6, 7, 8 and 9 are arranged about the electron beam tube 10.

The polarities may be as indicated but would be reversed in the case of the second magnet assembly. FIG. 3a shows the path of an electron initially parallel with the axis through first a focusing and then a defocusing section. In the focusing section the electron is deflected towards the axis and its mean distance from the axis in the defocusing section is less than in the focusing section so that the net focusing force is less than the net focusing force and there is a resultant focusing action. In the case of 3b the sections are reversed and the initial defocusing section causes the electron to be deflected from the axis so that it passes through the focusing section at a greater mean distance from the axis than in passing through the defocusing section and this again results in a net focusing action.

Referring now to FIG. 4, each of the two focusing systems 11, 12 therein disclosed is similar to that described hereinbefore, except that the first alternating gradient focusing system 11, which may be either of the magnetic type or of the electrostatic type, is of relatively long focal length and of relatively small angular aperture, whereas the second alternating gradient focusing system 12 is of relatively short focal length and of larger angular aperture. The second focusing system 12 is placed in the neighborhood of the focal point of the first focusing system 11. By the focal point of the first focusing system 11 we mean that point at which the spot size in the plane of the second focusing system 12 varies symmetrically about a minimum for the maximum useful energy range.

Referring now to FIGS. 5 and 6, each alternating gradient focusing system 11, 12 may be of simplified construction as therein shown. Instead of using a pair of complete and separate quadrupole magnets, it is possible to use simply four permanent magnets 13 of cylindrical form. While in theory the magnets 13 should be of hyperbolic cross section, it is apparent from FIG. 6 that a circular magnet approximates the hyperbolic form to a sufficient degree for practical purposes. The radius of the cylinder should be .7 times the minimum distance between opposite magnets. The set of four cylindrical permanent magnets 13 shown in FIGS. 5 and 6 correspond therefore to the two lenses 2 and 2 shown in FIG. 1. The four magnets 13 are enclosed in a non-magnetic tube 14 and magnetic shims 15 are provided at the extremities of the magnets 13 on the outside of the non-magnetic tube 14 in order to vary the focal length of the focusing system. Movement of these magnetic shims 15 thus provides a simple method of adjusting the focal length of the alternating gradient focusing system. Adjustment of this foral length is particularly important in the embodiment of the invention shown in FIG. 4, where two alternating gradient focusing systems 11, 12 are involved.

Having thus described the principles of the invention together with several illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

We claim:

1. Apparatus for obtaining a fine focal spot comprising an alternating gradient magnetic focusing system of relatively long focal length and of relatively small angular aperture, and in combination therewith an alternating gradient focusing system of relatively short focal length and of larger angular aperture, said second focusing system being placed in the neighborhood of the focal point of said first focusing system, wherein the term the focal point of the first focusing system means that point at which the spot size in the plane of the second focusing system varies symmetrically about a minimum for the maximum useful energy range.

2. Apparatus for obtaining a fine focal spot comprising an alternating gradient electrostatic focusing system of relatively long focal length and of relatively small angular aperture, and in combination therewith an alternating gradient focusing system of relatively short focal length and of larger angular aperture, said second focusing system being placed in the neighborhood of the focal point of said first focusing system, wherein the term the focal point of the first focusing system means that point at which the spot size in the plane of the second focusing system varies symmetrically about a minimum for the maximum useful energy range.

References Cited by the Examiner UNITED STATES PATENTS 2,849,634 8/58 Crowley-Milling 313-57 GEORGE N. WESTBY, Primary Examiner.

RALPH P. NILSON, Examiner. 

1. APPARATUS FOR OBTAINING A FINE FOCAL SPOT COMPRISING AN ALTERNATING GRADIENT MAGNETIC FOCUSING SYSTEM OF RELATIVELY LONG FOCAL LENGTH AND OF RELATIVELY SMALL ANGULAR APERTURE, AND IN COMBINATION THEREWITH AN ALTERNATING GRADIENT FOCUSING SYSTEM OF RELATIVELY SHORT FOCAL LENGTH AND OF LARGER ANGULAR APERTURE, SAID SECOND FOCUSING SYSTEM BEING PLACED IN THE NEIGHBORHOOD OF THE FOCAL POINT OF SAID FIRST FOCUSING SYSTEM, WHEREIN THE TERM "THE FOCAL POINT OF THE FIRST FOCUSING SYSTEM" MEANS THAT POINT AT WHICH THE SPOT SIZE IN THE PLANE OF THE SECOND FOCUSING SYSTEM VARIES SYMMETRICALLY ABOUT A MINIMUM FOR THE MAXIMUM USEFUL ENERGY RANGE. 